Antiretroviral drug resistance is becoming increasingly recognized as a critical determinant in the durability of efficacy of chemotherapy for HIV Infection. The complexity of mutations that emerge in both the genes for reverse transcriptase and protease is also being increasingly appreciated. The aim of the study is to determine whether variation in the outcome of therapy can be explained by variation in the genetic background in the pol gene present at the outset of therapy or in the order and identity of mutations that arise during treatment. Outcome will be analyzed as both virological (IC 50 of isolate, plasma HIV RNA) and clinical measures. A large data set of nevirapine treated patients with differing outcomes have been studies. Sequence analysis will be conducted by a variety of approaches. The results of these analyses will then drive the investigation of additional specimens and the design of additional trials to address the aims of the study.